There is a long felt need for an economically viable process to convert acetic acid to ethyl acetate. Ethyl acetate is an important commodity feedstock for a variety of industrial products and is also used as an industrial solvent in the manufacture of various chemicals. For instance, ethyl acetate can readily be converted to ethylene by subjecting it to a cracking process, which can then be converted to a variety of other products. Ethyl acetate is conventionally produced from feedstocks where price fluctuations are becoming more significant. That is, fluctuating natural gas and crude oil prices contribute to fluctuations in the cost of conventionally produced, petroleum or natural gas-sourced ethyl acetate, making the need for alternative sources of ethyl acetate all the greater when oil prices rise.
Ethanol is another important commodity chemical, which may be used in its own right, for example as a fuel, or as a feedstock for forming ethylene, vinyl acetate, ethyl acetate, or other chemical products. The hydrogenation of carboxylic acids over heterogeneous catalysts to produce alcohols is well reported. For instance, U.S. Pat. No. 2,607,807 discloses that ethanol can be formed from acetic acid over a ruthenium catalyst at extremely high pressures of 700-950 bar in order to achieve yields of around 88%, whereas low yields of only about 40% are obtained at pressures of about 200 bar. However such extreme reaction conditions are unacceptable and uneconomical for a commercial operation.
More recently, even though it may not still be commercially viable it has been reported that ethanol can be produced from hydrogenating acetic acid using a cobalt catalyst at superatmospheric pressures such as about 40 to 120 bar. See, for example, U.S. Pat. No. 4,517,391 to Shuster et al.
On the other hand, U.S. Pat. No. 5,149,680 to Kitson et al. describes a process for the catalytic hydrogenation of carboxylic acids and their anhydrides to alcohols and/or esters utilizing a platinum group metal alloy catalyst. The catalyst is comprised of an alloy of at least one noble metal of Group VIII of the Periodic Table and at least one metal capable of alloying with the Group VIII noble metal, admixed with a component comprising at least one of the metals rhenium, tungsten or molybdenum. Although it has been claimed therein that improved selectivity to a mixture of alcohol and its ester with the unreacted carboxylic acid is achieved over the prior art references it was still reported that 3 to 9 percent of alkanes, such as methane and ethane are formed as by-products during the hydrogenation of acetic acid to ethanol under their optimal catalyst conditions.
A slightly modified process for the preparation of ethyl acetate by hydrogenating acetic acid has been reported in EP 0 372 847. In this process, a carboxylic acid ester, such as for example, ethyl acetate is produced at a selectivity of greater than 50% while producing the corresponding alcohol at a selectivity less than 10% from a carboxylic acid or anhydride thereof by reacting the acid or anhydride with hydrogen at elevated temperature in the presence of a catalyst composition comprising as a first component at least one of Group VIII noble metal and a second component comprising at least one of molybdenum, tungsten and rhenium and a third component comprising an oxide of a Group IVB element. However, even the optimal conditions reported therein result in significant amounts of by-products including methane, ethane, acetaldehyde and acetone in addition to ethanol. In addition, the conversion of acetic acid is generally low and is in the range of about 5 to 40% except for a few cases in which the conversion reached as high as 80%.
From the foregoing it is apparent that existing processes do not have the requisite selectivity to ethyl acetate and/or ethanol, employ highly expensive catalysts or produce undesirable by-products such as methane and ethane. Thus, the need exists for forming ethyl acetate (and optionally ethanol) at high selectivity using a more economical catalyst, while minimizing the formation of undesirable byproducts.